Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a process of producing IC. LSI etc., in order to provide reliable semiconductor connections for various coatings used in manufacturing the circuits. In the step of polishing finish, a polishing pad is generally stuck on a rotatable supporting disk called a platen, while a workpiece such as a semiconductor wafer is stuck on a polishing head. By movement of the two, a relative speed is generated between the platen and the polishing head while polishing slurry having abrasive grains is continuously supplied to the polishing pad, to effect polishing processing.
As polishing characteristics of a polishing pad, it is requested that an object to be polished is excellent in planarity and in-plane uniformity and a polishing rate is large. A planarity and in-plane uniformity of an object to be polished can be improved to some extent with a polishing layer higher in elastic modulus. A polishing rate can be bettered by increasing a holding quantity of a slurry on a foam with cells therein.
Polishing pads including a polyurethane foam are proposed as polishing pads that meet the above properties (see Patent Documents 1 and 2). Such a polyurethane foam is produced by a reaction of an isocyanate-terminated prepolymer with a chain extender (curing agent), in which in view of hydrolysis resistance, elastic properties, wear resistance, or the like, a polyether (a polytetramethylene glycol with a number average molecular weight of 500 to 1,600) or a polycarbonate is preferably used as a high molecular polyol component for the isocyanate prepolymer.
However, the polishing pad has a problem such that the hardness is reduced at moisture or water absorption and the planarization characteristics thereby deteriorate gradually.
Moreover, in order to suppress the reduction in hardness of the polishing pad at moisture or water absorption, if the elastic modulus of the polishing pad is increased, the service life tends to be reduced due to deterioration in wear resistance. Thus, it has been difficult to achieve a balance between the wear resistance and the suppression of reduction in the hardness of the polishing pad.
In order to obtain a polyurethane elastomer cast product excellent in wear resistance and mechanical property, Patent Document 3 proposes a polyurethane elastomer composition for casting, containing a curing agent (liquid B) and an NCO group-terminated urethane prepolymer (liquid A) that is obtained by respectively using tolylene diisocyanate as an isocyanate component, a polytetramethylene ether glycol having a number average molecular weight of 500 to 4000 and a low molecular weight glycol having no side chain as glycol components and reacting these compounds.
Patent Document 4 proposes a polishing pad having a polishing layer including a polyurethane resin foam having fine cells, wherein a high molecular weight polyol component that is a starting component of the polyurethane foam contains a hydrophobic high molecular weight polyol A having a number average molecular weight of 550 to 800 and a hydrophobic high molecular weight polyol B having a number average molecular weight of 950 to 1300 in an A/B ratio of from 10/90 to 50/50 by weight. Patent Document 4 also describes that the polishing pad is excellent in polishing rate and superior in service life characteristics without generating center slow.
Patent Document 5 proposes a polishing pad having a polishing layer made of non-foamed polyurethane, wherein the non-foamed polyurethane is a reaction cured body of a polyurethane starting material composition which contains an isocyanate-terminated prepolymer obtained by the reaction of a prepolymer starting material composition containing a diisocyanate, a high molecular weight polyol, and a low molecular weight polyol; an isocyanate degenerated body which is polymerized by the addition of three or more diisocyanates; and a chain extender, wherein the addition amount of the isocyanate degenerated body is 5 to 30 parts by weight based on 100 parts by weight of the isocyanate-terminated prepolymer. It is also stated in Patent Document 5 that the polishing pad hardly causes scratches on the surface of an object to be polished and is excellent in dressing properties.